Aqueous binders for heat-sealing applications

ABSTRACT

The present invention relates to aqueous binders and to heat-sealing lacquers produced therefrom for heat-sealable coatings which adhere on aluminium without use of any primer, permit good sealability of the coated aluminium foil with respect to PS and/or PVC, and moreover feature good blocking resistance even at temperatures above 40° C.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. Non-Provisional applicationSer. No. 14/418,288, which was filed on Jan. 29, 2015. U.S.Non-Provisional application Ser. No. 14/418,288 is a National Stage ofPCT/EP2013/068471, which was filed on Sep. 6, 2013. This application isbased upon and claims the benefit of priority to German Application No.10 2012 218 108.3, which was filed on Oct. 4, 2012.

FIELD OF THE INVENTION

The present invention relates to aqueous binders and to heat-sealinglacquers produced therefrom for heat-sealable coatings which adhere onaluminium without use of any primer, permit good sealability of thecoated aluminium foil with respect to PS and/or PVC, and moreoverfeature good blocking resistance even at temperatures above 40° C.

It is now already some years since heat-sealing systems for the coatingof aluminium fools with respect to PS surfaces, PVC surfaces or PPsurfaces became established coating compositions in the food-and-drinkindustry. Heat-sealing coatings of this type are found on yoghurt potlids and also on the inner side of blisters, e.g. for medicaments. Mostestablished systems are based on organic solutions or organicdispersions. It is therefore already some years since interest began inthe development of an aqueous heat-sealing lacquer.

PRIOR ART

U.S. Pat. No. 6,194,514 explicitly describes aqueous two-layer systemscomposed of a primer on aluminium and of a second layer of an aqueousbinder on the dried primer. An example of a primer described is anaqueous poly(meth)acrylate dispersion which is composed of twocopolymers. One of the said copolymers here comprises from 2 to 10% byweight of glycidyl- or hydroxyl-functional monomers, and the othercopolymer comprises from 2 to 10% by weight of acrylic or itaconic acid.EP 0 417 570 discloses an aqueous copolymer dispersion for the sealingof abovementioned materials. This copolymer is composed of at least 70%by weight of methacrylates with C1- to C4-alkyl esters, and of at leastone functional monomer from the group of the acrylamides, methamlamides,aminoalkyl acrylates or aminoalkyl methacrylates. Although these systemsexhibit excellent heat sealing strengths, they function exclusively astwo-layer systems and need markedly longer staving the lacquers thanestablished solvent-based systems.

DE 3921256 describes an aqueous polymer dispersion as heat-sealinglacquer. Here, secondary dispersions are produced from solution polymerswith addition of water and ammonia and with subsequent distillativeremoval of the solvent. This process is very complicated. It is moreoververy difficult to achieve realty complete removal of the solventresidues, and this can in particular be disadvantageous in applicationsin contact with food or drink.

EP 0 574 603 describes aqueous dispersions for sealable coatings whichcomprise two copolymers with glass transition temperatures of from 50 to150° C. and from −50 to 50° C. One of the two copolymers here has from 3to 70% by weight content of acid-functional monomers. However, thesesystems do not have a good combination of blocking resistance andheat-seal-seam strength. The expression blocking resistance here meansthe undesired adhesion of the heat-sealing lacquer to a second aluminiumfoil or to a second lacquer-coated aluminium foil at temperatures belowthe sealing temperature. Blocking resistance is significant inparticular in relation to the storage of coated aluminium foils.

WO 2011/017388 describes an aqueous dispersion for heat-sealablecoatings, comprising a first copolymer with a glass transitiontemperature T_(g) of from −60 to 0° C., and from 0.2 to 10% by weight ofan ethylenically unsaturated acid or anhydride thereof. A second, hardcopolymer with a glass transition temperature of from 50 to 120° C. isalso present. Again, these systems do not have an adequate combinationof blocking resistance and heat-seal-seam strength.

U.S. Pat. No. 6,368,707 describes heat-sealable substrates composed of acopolymer dispersion in which a (meth)acrylate-based polymer with aglass transition temperature of at most −10° C. produced by, means ofemulsion polymerization is present with a copolymer with a glasstransition temperature above 20° C., which is a water-soluble orwater-dispersible copolymer having carboxy groups neutralized by a base.There is no description of sealability in respect of aluminium.

Object

In the light of the prior art, it was an object of the present inventionto develop a novel process for the coating of aluminium foils with anaqueous binder dispersion as heat-sealing lacquer and for the sealing ofthe aluminium foil with respect to PS or PVC.

In particular, it was an object of the present invention to develop acorresponding binder with which heat-seal-seam strengths of at least 5N/15 mm can be achieved after application without primer.

Very particularly, it as an object of the present invention to provide aheat-sealing process which uses aqueous dispersions and which exhibitsreasonable drying times and which after sealing, exhibits good blockingresistance.

Another object of the present invention was to develop a process whichuses an aqueous binder dispersion which is free from organic solventsand which can be produced without the use of such solvents. Inparticular, it was an object of the present invention to provide anaqueous binder dispersion which also functions without addition ofammonia.

A particular object of the invention was that when aluminium is sealedwith respect to polystyrene the fracture site is on the polystyreneside.

Other objects not explicitly mentioned can be derive description and theexamples, or directly from the prior art.

Achievement of Objects

The objects are achieved via the novel use of aqueous dispersion in aheat-sealing lacquer for the sealing of aluminium surfaces which respectto styrene, PET, PLA (polylactic acid) or PVC. This heat-sealing lacqueris composed of at least 50% by weight, preferably from 70 to 95% byweight, of the said aqueous dispersion.

This aqueous dispersion used according to the invention comprises afirst polymer phase with a glass transition temperature of from −20 to30° C., preferably from −10 to 25° C., particularly preferably from −5to,5° C., and a second polymer phase with a glass transition temperatureof from 20 to less than 50° C., preferably from 30 to 45° C. The secondpolymer phase here comprises, based on the entirety of the two polymerphases, from 2 to 10% by weight of acids copolymerizable withmethacrylates.

It is preferable that the heat-sealing lacquer is applied in a singlelayer to the aluminium surface before the sealing process. The aqueousdispersion present here preferably comprises from 15 to 64% by weight ofa polymer or polymers. These in turn preferably comprise from 25 to 78%by weight, particularly from 37 to 70% by weight, in particular from 49to 65% by weight, of an alkyl ester of metharrylic acid, from 4 to 40%by weight, particularly from 12 to 35% by weight and in particular from17 to 30% by weight, of an alkyl ester of acrylic acid, from 2 to 9% byweight, particularly from 3 to 8% by weight, of an acid which iscopolymerizable with methacrylates and which preferably involves(meth)acrylic acid, and up to 20% by weight, particularly from 4 to 20%by weight and very particularly from 8 to 15% by weight, of anothermonomer which is copolymenzable with (meth)acrylates but which is notitself a (meth)acrylate, and which preferably involves styrene.

The polymer(s) in the dispersion can moreover also comprise from 2 to12% by weight, preferably from 3 to 7% by weight and particularlypreferably from 4 to 6% by weight, of an alkyl ester of (meth)acrylicacid having at least one other functional group. This alkyl ester of(meth)acrylic acid having at least one other functional group preferablyinvolves hydroxyethyl (meth)acrylate.

In this connection, the expression (meth)acrylic acid means methacrylicacid, acrylic or a mixture of these. Corresponding considerations applyto the expression (meth)acrylate, which comprises either methacrylatesor acrylates or a mixture of these. In contrast, each of the expressionsacrylate and methacrylate means precisely that.

In particular, the present invention provides the use of an aqueousdispersion in a heat-sealing lacquer where the entirety of the polymersin the said dispersion are composed of from 25 to 78% by weight,preferably from 37 to 70% by weight, particularly preferably from 49 to65% by weight, of methyl methacrylate and/or butyl methacrylate, from 4to 40% by weight, particularly preferably from 12 to 35% by weight, withparticular preference from 17 to 30% by weight, and very particularlypreferably from 20 to 30% by weight, of a C₁-C₄-alkyl ester of acrylicadd, from 3 to 7% by weight, preferably from 4 to 6% by weight, of ahydroxy-functional (meth)acrylate which with particular;preferenceinvolves hydroxyethyl acrylate, from 2 to 9% by weight, preferably from5 to 7% by weight, of (meth)acrylic acid, which particularly preferablyinvolves a mixture of acrylic acid and methacrylic acid, and from 4 to20% by weight, preferably from 8 to 15% by weight, of styrene.

It is preferable that the aqueous dispersion is produced by means ofemulsion polymerization. A first monomer mixture here which leads to apolymer with a glass transition temperature of from −20 to 30° C. isused as initial charge and, after polymerization of this monomermixture, a second monomer mixture which leads to the polymer with glasstransition temperature of from 20 to less than 50° C. added, andpolymerized.

It s particularly preferable that the first monomer mixture herecomprises the hydroxy-functional (meth)acrylates and that the secondmonomer mixture comprises the carboxylic acids copolymerizable with(meth)acrylates. The ratio by weight here of the two monomer mixtures toone another is from 1:9 to 8:2.

In particular, the second monomer mixture can be polymerized tocompletion with the aid of a chain-transfer agent, preferably 0.2% byweight of n-DDM (n-dodecyl mercaptan), based on the monomers.

In particular, the emulsion polymerization is carried out in an aqueousphase in the presence of anionic, cationic or non-ionic emulsifiersknown per se and of at least one free-radical-generating initiator in atwo-stage reaction. The conduct of this type of emulsion polymerizationcan be found by way of example in H. Rauch-Puntigam, Th. VölkerAcryl-und Methacrylverbindungen [Acrylic and methacrylic compounds],Springer-Verlag 1967, pp. 217-230. The first polymerization stage hereis carried out by means of a feed process where, once a certaintemperature has been reached, the initiator, in particular inorganicperoxides such as potassium peroxodisulphate or ammoniumperoxodisulphate (PPS, APS), preferably dissolved in water, is added toan initial charge made of deionized water and emulsifier in a suitablereaction vessel equipped with stirrer and heating system. The content ofinitiator in the initial charge is by way of example in the range from0.01 mol % to 2 mol %, based on the monomers in the feed in the firststage.

Amounts used of ionic emulsifiers are from 0.01 to 2.0% by weight, basedon the entire amount of monomer, and those that can be used are inparticular anionic emulsifiers. An example here is AEROSOL OT75(R) fromCyanamid BV, REWOPOL SB DO 75 from Evonik Tego Chemie GmbH or Dowfax 2A1from Dow Europa SA. The mixture characterized as feed above is addedthereto over a certain period, e.g. within 2 hours, in a first stage ofthe feed process. By way of example, feed 1 comprises from 20 to 60% byweight of the entire amount of water together with the monomers of thefirst stage.

After first-stage polymerization, stirring can also be continued for acertain period, for example for a period of one hour, at elevatedtemperature, e.g. at 80° C. The mixture can then be cooled, for exampleto 30° C., before feed of the second monomer composition is begun. Thissecond feed can by way of example proceed over a period of 30 min. Itcan then be advantageous to allow the mixture to swell over a prolongedperiod, by way of example 4 hours, before heating to the secondpolymerization temperature of, for example, 40° C. and again addinginitiators. It is preferable here to use redox initiators. Examples hereare peroxodisulphates, dithionites and iron sulphate. After theaddition, an exothermic reaction is observed, and once this stage hasbeen reached the temperature is raised, e.g. to 80° C. Finally, stirringis continued by way of example for 2 hours in order to complete thepolymerization. As an alternative, the second polymerization stage canalso be a feed polymerization, e.g. over a period of 2 hours.

Another possibility, as an alternative to the use of ionic emulsifiers,is the addition of from 0.01 to 5.0% by weight of non-ionic emulsifiers,e.g. ethoxylated alcohols or methacrylic esters of methoxy polyethyleneglycols, for example Carbowax 550, or alkylphenols. It is also possibleto use a combination of ionic and non-ionic emulsifiers.

In an optional use of the dispersion of the invention, small amounts ofammonia are added thereto in order to adjust pH before the sealingprocess. This embodiment can achieve improved colloidal stability andimproved coating properties.

The resultant dispersion can generally be used directly as it stands forthe coating process. In particular instances, limited amounts ofthickeners, antiblocking agents or film-formation aids can be added. Thecoating can be applied by spraying, spreading, casting, dip-coating,doctoring or rolling. Application thickness on the aluminium foil isgenerally such that drying gives a layer of from 2 to 10 μm.

Another possibility is the use of the aqueous dispersion of theinvention with addition of an organic solution made of apolymethacrylate, e.g. in, the form of a dissolved suspension polymer,e.g. DEGALAN® P 24 from Evonik industries. Another alternative is theaddition of aqueous methacrylate dispersions according to EP 0 417570A1, for example DEGALAN® 4032 D from Evonik industries, to thedispersion of the invention.

In another possible application, the dried coating produced by means ofthe dispersion of the invention can also be coated with a layer of abinder according to EP 0 417 570A1, e.g. DEGALAN® 4032 D, for a furtherimprovement in blocking resistance and water resistance. However, thisoptional application is not preferred because this procedure would losethe advantage of a single-layer heat-sealing layer, and the dispersionsof the invention intrinsically give very good heat-sealing properties.

In practice, the drying of the coated aluminium foil is advantageouslyundertaken in a drying oven or in a continuous drying tunnel, optionallyat reduced pressure and at temperatures of from 100 to 240° C. Therequired drying time generally becomes shorter as the drying temperatureincreases, and is by way of example from 5 sec to 5 min. Multipleapplication is, optionally also possible. The aqueous dispersions arepreferably suitable for application to on-porous, coherent substratesurfaces, for example those of plastics foils or in particular of metalfoils, for example aluminium foils or iron.

Heat-sealing generally requires that the glass transition temperature ofthe poly(meth)acrylate is exceeded in the coating. The shorter thecontact time and the poorer the heat transfer through the substratelayer, the greeter the extent to which the temperature of theheat-sealing process has to be above the required sealing temperature.Thin metal foils exhibit very good heat transfer and allow thetemperature of the heat-sealing jaws to be only slightly above themelting point of the poly(meth)acrylate, although in practice markedlyhigher heat-sealing temperatures are mostly selected, e.g. from 100 to240° C., in order to maximize sealing rate. The glass transitiontemperature of a plastics substrate can restrict the heat-sealingtemperature. In order to achieve high-strength sealing, pressure of atleast 1 kp/cm², preferably from 3 to 6 kp/cm², should be applied duringthe heat-sealing process.

For the purposes of the present invention, the seal seam strength of theheat-sealing specimens is tested in accordance with DIN 51 221.

The examples below serve to illustrate the invention.

EXAMPLES

Production specifications for Inventive Examples 1-11 and ComparativeExamples 1 and 3.

Inventive Example 9 provides a detailed description of the synthesismethod and of the nature of the starting materials used. InventiveExample 9 differs from Inventive Examples 1-8, inventive Examples 10 and11, and also Comparative Examples 1 and 3, in monomer composition of thefirst and second stage, which can be found in table 1, and also in thedistribution of some of the starting materials, amounts of which can becalculated as follows: all of the polymerization processes wereconducted in two stages, in each case distributing 520 g of monomer overthe two stages. Emulsifier content is 0.52%, of which 52.4% are used inthe first stage and 47.6% in the second stage. 10% of the emulsifier forthe first stage here are used in the initial charge in the reactor, and90% are used in the emuision. The emulsions are in each case mixed with34% by weight water content. The amount of initiator used is 0.09505 mol% of ammonium persulphate (APS), based on the monomers of the firststage. A further amount of 0.1062 mol % of initiator, based on themonomers of the second stage, is added to the second-stage emulsion.

Production Specification for Inventive Example 9

224 g of deionized water and 0.19 g of Rewopol SBDO 75 emulsifier areweighed into a 1 litre round-bottomed Quickfit flask with Quickfit lid,thermometer and stirrer, and are heated to an internal temperature ofabout 80° C. in a water bath, with stirring (150 rpm). The first-stageemulsion was produced by, weighing 1.70 g of Rewopol SBDO 75, 36.40 g ofhydroxyethyl acrylate, 162.0 g of MMA, 165.6 g of n-butyl acrylate and188.0 g of deionized water into a Woulff bottle and stirring thismixture for 5 min, leaving it to stand for 1 min and then stirring for afurther 15 min.

The initial charge in the reactor is heated to an internal temperatureof 80° C. and then 7.0 mL of APS (10% by weight) are added andincorporated by stirring for 5 min. The emulsion is metered at ametering rate of 3.3 g/min for three minutes. A slight temperature riseoccurs here. and the metering is interrupted for 4 min. The rest of theemulsion is now metered at a metering rate of 3.3 g/min, and oncompletion stirring continued for 20 min.

The second-stage em ion is produced by weighing 1.72 g of Rewopol 580075 emulsifier, 15.6 g of acrylic acid, 31.2 g of styrene, 109.2 g ofn-butyl methacrylate and 81 g of deionized water into a Woulff bottle,stirring the mixture for 5 min, leaving it to stand min and then againstirring for 15 min. 3.1 g of ammonium persulphate are added to thismixture and incorporated by vigorous stirring.

Once the reaction time for the first stage has expired, the second stageis metered into ;.a mixture at a metering rate of 3.3 g/min, and this isfollowed by 60 minutes of continued-reaction time. The dispersion iscooled and then filtered through a 150 μm sieve.

Production Specification for Comparative Example CE2

Comparative Example CE 2 was produced in accordance with the prior artof WO2011017388. Example 2.

Foil Material Used

High-flexibility aluminium foils of thickness 38 μm and PS and PVC foilsof thickness 500 μm were used.

Laboratory Application of Heat-Sealing Dispersion

A K Hand coater No. 3 was used to apply the aqueous binder.

Laboratory Drying of Coated Foils

Directly after application of the aqueous binder, the foils were driedat 180° C. in convection oven for 15 seconds.

Heat-Sealing and Determination of Seal Seam Strength

Heat sealing equipment from LOWA GmbH was used to produce the seals.

Sealing condition

Temperature: 180° C.

Pressure: 3 bar

Time: 1 sec.

Sealing area: 10×100 mm

Seal seam strength was determined by cutting specimens into strips ofwidth 15 mm and subjecting these t tension at a velocity of 100 mm/min.,in an Instron 1195 or Zwick 1454 tensile tester. During tensile testing,care as taken to ensure that the angle between the separated parts ofthe foils and the remainder not yet subjected to stress was 90°.

Exposure to Water

The water resistance of the lacquer was determined by placing the sealedstrips in mains water for 48 h, and then drying them and determiningheat sealing strength as described above.

Determination of Blocking Point

Blocking point was determined by using the heat-sealing equipmentdescribed above, but after replacement of one of the heated jaws by anunheated rubber jaw. The lacquered sides of two lacquered aluminiumstrips (prepared as described above) were pressed against one another ata defined temperature under a pressure of one bar for 30 seconds in theequipment. The blocking point is the temperature at which the aluminiumstrips remain adhering to one another when only one of the strips isheld. At lower the weight of the aluminium strips is sufficient toseparate these from one another. Measurements were made at intervals of5° C.

TABLE 1 Compositions and production process for Inventive Examples 1-9Proportion of Stage 1 Stage 2 Specimen Stage 1 in % MMA n-BA HEA BMA AAStyrene n-DDM TGA Inv. Ex. 1 50 57 33 10 70 10 20 Inv. Ex. 2 60 44.545.5 10 69.8 10 20 0.2 Inv. Ex. 3 50 44.5 45.5 10 70 10 20 Inv. Ex. 4 6044.5 45.5 10 80 10 10 Inv. Ex. 5 60 44.5 45.5 10 69.8 10 20 0.2 Inv. Ex.6 50 48 52 70 10 20 Inv. Ex. 7 50 48 42 10 70 10 20 Inv. Ex. 8 30 44.545.5 10 70 10 20 Inv. Ex. 9 70 44.5 45.5 10 70 10 20 MMA: Methylmethacrylate; n-BA: n-butyl acrylate; HEA: 2-hydroxyethyl acrylate; BMA:n-butyl methacrylate; AA: acrylic acid; n-DDM: n-dodecyl mercaptan; TGA:thioglycolic acid

TABLE 2 Composition and production process for Comparative Examples 1-3and Inventive Examples 10-11 Proportion of Stage 1 Stage 2 SpecimenStage 1 in % MMA BMA EA AA BA HEA MMA BMA Styrene EHMA AA CE1 50 44.545.5 10 70 10 10 10 CE2 see WO2011017388, Example 2 CE3 30 70 20 10 7010 20 IE10 60 24 66 10 70 10 20 IE11 15 44.5 0 45.5 10 0 70 20 0 10 MMA:methyl methacrylate; EA: ethyl acrylate; BA: n-butyl acrylate; HEA:2-hydroxyethyl acrylate; BMA: n-butyl methacrylate; AA: acrylic acid;n-DDM: n-dodecyl mercaptan; TGA: thioglycolic acid; EHMA: ethylhexylmethacrylate

TABLE 3 Properties of binders produced in Inventive Examples 1-11 andComparative Examples 1-3 HSF v PS [N/15 mm] HSF v PVC [N/15 mm] H₂O H₂OBlocking Tg Tg Fracture Fracture Fracture Fracture point Ps Stage 1Stage 2 Specimen site site site site [° C.] [nm] [° C.] [° C.] IE 1 6 PSnd nd <1 partial nd nd 50 146 20  41 IE 2 8 PS 8 PS 7 Al 8 partial 45161 0 41 IE 3 7 PS nd nd 6 Al nd nd 45 172 0 41 IE 4 7 PS nd nd 6 Al ndnd 40 145 0 34 IE 5 8 PS 8 partial 6 Al 5 partial 45 143 0 41 IE 6 2 PSnd nd 5 Al nd nd 45 150 0 41 IE 7 7 PS nd nd 5 partial nd nd 45 124 5 41IE 8 7 PS 7 PS 2 partial 6 partial 50-55 181 0 41 IE 9 8 PS 7 PS 6 Al 4Al 45 153 0 41 IE 10 9 PS 6 Al 6 Al 5 Al 40 141 1 41 IE 11 4 PS 5 PS 1nd 1 nd 55 263 0 41 CE1 1 PS nd nd 1 PVC nd nd 55-60 158 0 79 CE2 6 Both7 partial 5 partial 6 partial <35  78 −15*   56* sides CE3 3 Al 1partial 60 155 43  41

Glass transition temperatures indicated by “*” have been taken fromWO2011017388. The “HSF” column in the table states the heat sealingstrength values measured with respect to polystyrene (PS) and polyvinylchloride (PVC). The column headed “H₂O” describes the heat sealingstrength values after exposure to water. The “Blocking point” columnindicates the blocking points measured. “Ps” describes the particle size(determined with Beckmann Coulter LS 13320 equipment, the stated valuesbeing the d50 from the numerical distribution), and T_(g) (stage 1) andT_(g) (stage 2) are the calculated glass transition temperatures of therespective stages. T_(g) is calculated by using the Fox equation.

Inventive Examples 1-9 and 11 comprise dispersions with identicalmonomer units but markedly different compositions within the individualstages, and in particular with markedly different stage 1: stage 2ratios. Nevertheless, despite these wide variations good heat sealingstrengths and good blocking resistance values are obtained throughout.In the case of inventive Example 11—with only a small proportion ofstage 1—strength values observed with respect to PS after exposure towater are still surprisingly good. Inventive Example 10 shows that it isnot essential to use butyl acrylate and that this monomer can bereplaced for example by ethyl acrylate, as long as the necessaryalterations, which will be understood by the person skilled in the art,are made in the ratios of the individual monomers. However, use ofmonomers with longer side chains, e.g. n-butyl acrylate or ethylhexylacrylate, can be expected to give inter aim better water resistancevalues for the lacquers.

Comparative Example 1 corresponds to the systems of the prior art inaccordance with EP 0 574 803, and does not exhibit any significant heatsealing strength values.

Comparative Examples 2 corresponds to Example 2 from WO2011017388.Blocking resistance is inadequate.

Comparative Example 3 exhibits inadequate heat sealing strength values.The glass transition temperature of the first stage is too high.

1. A process, comprising applying a heat-sealing lacquer in a singlelayer to an aluminum surface prior to performing a sealing process,wherein the heat-sealing lacquer comprises at least 50% by weight of anaqueous dispersion comprising: a first polymer phase having a glasstransition temperature of from −20 to 30° C.; and a second polymer phasehaving a glass transition temperature of from 20 to less than 50° C. andcomprising from 2 to 10% by weight of at least one acid copolymerizablewith methacrylates, based on a total weight of the first polymer phaseand the second polymer phase.
 2. The process of claim 1, wherein theaqueous dispersion comprises from 15 to 64% by weight of polymerscomprising: from 25 to 78% by weight of an alkyl ester of methacrylicacid; from 4 to 40% by weight of an alkyl ester of acrylic acid; from 2to 9% by weight of the at least one acid copolymerizable withmethacrylates; and up to 20% by weight of at least one other monomercopolymerizable with (meth)acrylates, but which is not a (meth)acrylate.3. The process of claim 1, wherein polymers in the aqueous dispersioncomprise from 49 to 65% by weight of an alkyl ester of methacrylic acid;from 17 to 30% by weight of an alkyl ester of acrylic acid; from 3 to 8%by weight of (meth)acrylic acid; and from 8 to 15% by weight of styrene.4. The process of claim 1, wherein: the first polymer phase has a glasstransition temperature of from −10 to 25° C.; and the second polymerphase has a glass transition temperature of from 30 to 45° C.
 5. Theprocess of claim 1, wherein polymers in the aqueous dispersion furthercomprise from 2 to 12% by weight of an alkyl ester of (meth)acrylic acidhaving at least one other functional group.
 6. The process of claim 5,wherein the alkyl ester of (meth)acrylic acid having at least one otherfunctional group comprises hydroxyethyl (meth)acrylate, and an amount ofthe hydroxyethyl (meth)acrylate in the polymers is from 3 to 7% byweight.
 7. The process of claim 1, wherein polymers of the aqueousdispersion comprise: from 37 to 70% by weight of methyl methacrylate,butyl methacrylate, or both, from 12 to 35% by weight of a C₁-C₄-alkylester of acrylic acid; from 3 to 7% by weight of a hydroxy-functional(meth)acrylate; from 2 to 9% by weight of (meth)acrylic acid; and from 4to 20% by weight of styrene.
 8. The process of claim 7, wherein thepolymers comprise: from 49 to 65% by weight of the methyl methacrylate,the butyl methacrylate, or both; from 20 to 30% by weight of theC₁-C₄-alkyl ester of acrylic acid; from 4 to 6% by weight of thehydroxy-functional (meth)acrylate; from 5 to 7% by weight of the(meth)acrylic acid; and from 8 to 15% by weight of the styrene.
 9. Theprocess of claim 1, wherein the aqueous dispersion is formed by emulsionpolymerization, such that: a first monomer mixture, which forms apolymer having a glass transition temperature of from −20 to 30° C., isinitially charged; and after polymerization of the first monomer mixturea second monomer mixture, which forms a polymer having a glasstransition temperature of from 20 to less than 50° C. is added, andpolymerized, to form a core-shell particle comprising the first polymerphase and the second polymer phase.
 10. The process of claim 9, wherein:the first monomer mixture comprises one or more hydroxy-functional(meth)acrylates; the second monomer mixture comprises a carboxylic acidcopolymerizable with (meth)acrylates; and a ratio by weight of the firstmonomer mixture to the second monomer mixture ranges from 1:9 to 8:2.11. The process of claim 9, wherein the second monomer mixture ispolymerized in the presence of a chain-transfer agent.
 12. The processof claim 1, wherein the heat-sealing lacquer is effective to sealaluminum surfaces with respect to styrene, PET, PLA or PVC.
 13. Theprocess of claim 1, wherein: the first polymer phase has a glasstransition temperature of from −5 to 5° C.; and the second polymer phasehas a glass transition temperature of from 30 to 45° C.
 14. The processof claim 5, wherein the alkyl ester of (meth)acrylic acid having atleast one other functional group comprises hydroxyethyl (meth)acrylate,and an amount of the hydroxyethyl (meth)acrylate in the polymers is from4 to 6% by weight.
 15. The process of claim 9, wherein the secondmonomer mixture is polymerized in the presence of 0.2% by weight ofn-DDM as a chain-transfer agent.